The physical size of consumer devices has decreased significantly over the years and this trend is at odds with an ideal acoustic environment. This reduction in device size has necessitated the use of micro-loudspeakers, which are designed differently than their typical loudspeaker counterparts. One salient drawback of a micro-loudspeakers, as can be seen in FIG. 1, is their reduced low frequency response, which for more acceptable audio quality, requires a significant amount of flattening equalization (EQ). FIG. 1 is an example of a micro-loudspeaker frequency response, where speaker resonance (f0) is 845 Hz and the quality factor of the resonance in a closed box (Qtc) is 3.0.
The shape of such an equalization curve below 2 kHz typically contains at least three components: highpass, low-frequency boost, and peak resonance attenuation, as depicted in FIG. 2. FIG. 2 is an example of a resulting micro-loudspeaker frequency response, curve 139, with the flattening EQ, curve 137, comprised of highpass, low shelf, and peak filters applied. Although a static compensating equalization can dramatically improve the subjective and objective performance, at low or even moderate sound pressure level (SPL), the flattening equalization can cause distortions from loudspeaker over excursion at higher listening levels. For this reason there are several solutions in the market that provide a dynamic flattening equalization algorithm guided by a loudspeaker excursion model. These solutions essentially attempt to apply as much flattening equalization as possible without causing over excursion of the membrane or diaphragm within the loudspeaker. When measuring these solutions using common synthetic log swept sinusoidal (LSS) signals, stationary tones or even noise, these systems seem to work fairly well despite having high levels of measured total harmonic distortion (THD).
Alternatively, it has been observed on several portable devices using micro-loudspeakers, that multi-tonal signals having only moderate amounts of energy below the speaker resonance can exhibit significant amounts of what is commonly referred to as “rub and buzz” distortion. It is, however, unclear if the cause of the distortion is from over-excursion or other acoustic/mechanical issue within the speaker and enclosure. Determining this would require measurements of the excursion using a speaker enclosure modified with a clear panel exposing the internal loudspeaker and a laser vibrometer to observe the loudspeaker excursion during the distortion events. Also, the low frequency boost within typical loudspeaker flattening solutions exacerbates the issue because the excursion models guiding the time varying equalization may be blind to the circumstances causing the distortion, leaving the equalization curve over engaged.